The quantitative determination of analytes in biological fluids is useful in the diagnosis and treatment of physiological abnormalities. For example, determining the glucose level in biological fluids, such as blood, is important to diabetic individuals who must frequently check their blood glucose level to regulate their diets and/or medication.
Electrochemical systems have been used for this type of analysis. During the analysis, the analyte undergoes a redox reaction with an enzyme or similar species to generate an electric current that may be measured and correlated with the concentration of the analyte. A substantial benefit may be provided to the user by decreasing the time required for the analysis while supplying the desired accuracy and precision.
One example of an electrochemical sensor system for analyzing analytes in biological fluids includes a measuring device and a sensor strip. The sensor strip includes reagents to react with and transfer electrons from the analyte during the analysis and electrodes to pass the electrons through conductors that connect the strip with the device. The measuring device includes contacts to receive the electrons from the strip and the ability to apply a voltage differential between the contacts. The device may record the current passing through the sensor and translate the current values into a measure of the analyte content of the sample. These sensor systems may analyze a single drop of whole blood (WB), such as from 1-15 microliters (μL) in volume.
Examples of bench-top measuring devices include the BAS 100B Analyzer available from BAS Instruments in West Lafayette, Ind.; the CH Instrument Analyzer available from CH Instruments in Austin, Tex.; the Cypress Electrochemical Workstation available from Cypress Systems in Lawrence, Kans.; and the EG&G Electrochemical Instrument available from Princeton Research Instruments in Princeton, N.J. Examples of portable measuring devices include the Ascensia Breeze® and Elite® meters of Bayer Corporation.
The sensor strip may include a working electrode where the analyte undergoes electrochemical reaction and a counter electrode where the opposite electrochemical reaction occurs, thus allowing current to flow between the electrodes. Thus, if oxidation occurs at the working electrode, reduction occurs at the counter electrode. See, for example, Fundamentals Of Analytical Chemistry, 4th Edition, D. A. Skoog and D. M. West; Philadelphia: Saunders College Publishing (1982), pp 304-341.
The sensor strip also may include a true reference electrode to provide a non-variant reference potential to the measuring device. While multiple reference electrode materials are known, a mixture of silver (Ag) and silver chloride (AgCl) is typical due to the insolubility of the mixture in the aqueous environment of the analysis solution. A reference electrode also may be used as the counter electrode. A sensor strip using such a combination reference-counter electrode is described in U.S. Pat. No. 5,820,551.
The sensor strip may be formed by printing electrodes on an insulating substrate using multiple techniques, such as those described in U.S. Pat. Nos. 6,531,040; 5,798,031; and 5,120,420. One or more reagent layer may be formed by coating one or more of the electrodes, such as the working and/or counter electrodes. In one aspect, more than one of the electrodes may be covered by the same reagent layer, such as when the working and counter electrodes are coated by the same composition. In another aspect, reagent layers having different compositions may be printed or micro-deposited onto the working and counter electrodes using the method described in a U.S. provisional patent application filed Oct. 24, 2003, Application No. 60/513,817. Thus, the reagent layer on the working electrode may contain the enzyme, the mediator, and a binder while the reagent layer on the counter electrode contains a soluble redox species, which could be the same as the mediator or different, and a binder.
The reagent layer may include an ionizing agent for facilitating the oxidation or reduction of the analyte, as well as any mediators or other substances that assist in transferring electrons between the analyte and the conductor. The ionizing agent may be an analyte specific enzyme, such as glucose oxidase or glucose dehydrogenase, to catalyze the oxidation of glucose in a whole blood (WB) sample. The reagent layer also may include a binder that holds the enzyme and mediator together. Table I, below, provides conventional combinations of enzymes and mediators for use with specific analytes.
TABLE IAnalyteEnzymeMediatorGlucoseGlucose OxidaseFerricyanideGlucoseGlucose DehydrogenaseFerricyanideCholesterolCholesterol OxidaseFerricyanideLactateLactate OxidaseFerricyanideUric AcidUricaseFerricyanideAlcoholAlcohol OxidasePhenylenediamine
The binder may include various types and molecular weights of polymers, such as CMC (carboxyl methyl cellulose) and/or PEO (polyethylene oxide). In addition to binding the reagents together, the binder may assist in filtering red blood cells, preventing them from coating the electrode surface.
Examples of conventional electrochemical sensor systems for analyzing analytes in biological fluids include the Precision® biosensors available from Abbott in Abbott Park, Ill.; Accucheck® biosensors available from Roche in Indianapolis, Ind.; and OneTouch Ultra® biosensors available from Lifescan in Milpitas, Calif.
One electrochemical method, which has been used to quantify analytes in biological fluids, is coulometry. For example, Heller et al. described the coulometric method for whole blood glucose measurements in U.S. Pat. No. 6,120,676. In coulometry, the analyte concentration is quantified by exhaustively oxidizing the analyte within a small volume and integrating the current over the time of oxidation to produce the electrical charge representing the analyte concentration. In other words, coulometry captures the total amount of glucose within the sensor strip.
An important aspect of coulometry is that towards the end of the integration curve of charge vs. time, the rate at which the current changes with time becomes substantially constant to yield a steady-state condition. This steady-state portion of the coulometric curve forms a relatively flat plateau region, thus allowing determination of the corresponding current. However, the coulometric method requires the complete conversion of the entire volume of analyte to reach the steady-state condition. As a result, this method is time consuming and does not provide the fast results which users of electrochemical devices, such as glucose-monitoring products, demand. Another problem with coulometry is that the small volume of the sensor cell must be controlled in order to provide accurate results, which can be difficult with a mass produced device.
Another electrochemical method which has been used to quantify analytes in biological fluids is amperometry. In amperometry, current is measured during a read pulse as a constant potential (voltage) is applied across the working and counter electrodes of the sensor strip. The measured current is used to quantify the analyte in the sample. Amperometry measures the rate at which the electrochemically active species, and thus the analyte, is being oxidized or reduced near the working electrode. Many variations of the amperometric method for biosensors have been described, for example in U.S. Pat. Nos. 5,620,579; 5,653,863; 6,153,069; and 6,413,411.
A disadvantage of conventional amperometric methods is the non-steady-state nature of the current after a potential is applied. The rate of current change with respect to time is very fast initially and becomes slower as the analysis proceeds due to the changing nature of the underlying diffusion process. Until the consumption rate of the reduced mediator at the electrode surface equals the diffusion rate, a steady-state current cannot be obtained. Thus, for amperometry methods, measuring the current during the transient period before a steady-state condition is reached may be associated with more inaccuracy than a measurement taken during a steady-state time period.
The “hematocrit effect” provides an impediment to accurately analyzing the concentration of glucose in WB samples. WB samples contain red blood (RB) cells and plasma. The plasma is mostly water, but contains some proteins and glucose. Hematocrit is the volume of the RB cell constituent in relation to the total volume of the WB sample and is often expressed as a percentage. Whole blood samples generally have hematocrit percentages ranging from 20% to 60%, with ˜40% being the average.
In conventional sensor strips for determining glucose concentrations, glucose may be oxidized by an enzyme, which then transfers the electron to a mediator. This reduced mediator then travels to the working electrode where it is electrochemically oxidized. The amount of mediator being oxidized may be correlated to the current flowing between the working and counter electrodes of the sensor strip. Quantitatively, the current measured at the working electrode is directly proportional to the diffusion coefficient of the mediator. The hematocrit effect interferes with this process because the RB cells block the diffusion of the mediator to the working electrode. Subsequently, the hematocrit effect influences the amount of current measured at the working electrode without any connection to the amount of glucose in the sample.
WB samples having varying concentrations of RB cells may cause inaccuracies in the measurement because the sensor may not distinguish between a lower mediator concentration and a higher mediator concentration where the RB cells block diffusion to the working electrode. For example, when WB samples containing identical glucose levels, but having hematocrits of 20, 40, and 60%, are analyzed, three different glucose readings will be reported by a conventional sensor system based on one set of calibration constants (slope and intercept, for instance). Even though the glucose concentrations are the same, the system will report that the 20% hematocrit sample contains more glucose than the 60% hematocrit sample due to the RB cells interfering with diffusion of the mediator to the working electrode.
The normal hematocrit range (RBC concentration) for humans is from 20% to 60% and is centered around 40%. Hematocrit bias refers to the difference between the reference glucose concentration obtained with a reference instrument, such as the YSI 2300 STAT PLUS™ available from YSI Inc., Yellow Springs, Ohio, and an experimental glucose reading obtained from a portable sensor system for samples containing differing hematocrit levels. The difference between the reference and experimental readings results from the varying hematocrit levels between specific whole blood samples.
In addition to the hematocrit effect, measurement inaccuracies also may arise when the measurable species concentration does not correlate with the analyte concentration. For example, when a sensor system determines the concentration of a reduced mediator generated in response to the oxidation of an analyte, any reduced mediator not generated by oxidation of the analyte will lead to the sensor system indicating that more analyte is present in the sample than is correct due to mediator background.
In addition to the hematocrit and mediator background effects, other factors also may lead to inaccuracies in the ability of a conventional electrochemical sensor system to determine the concentration of an analyte in a sample. In one aspect, these inaccuracies may be introduced because the portion of the sensor strip that contains the sample may vary in volume from strip to strip. Inaccuracies also may be introduced when sufficient sample is not provided to completely fill the volume of the cap-gap, a condition referred to as under-fill. In other aspects, inaccuracies may be introduced into the measurement by random “noise” and when the sensor system lacks the ability to accurately determine temperature changes in the sample.
In an attempt to overcome one or more of these disadvantages, conventional sensor systems have attempted multiple techniques, not only with regard to the mechanical design of the sensor strip and reagent selection, but also regarding the manner in which the measuring device applies the electric potential to the strip. For example, conventional methods of reducing the hematocrit effect for amperometric sensors include the use of filters, as disclosed in U.S. Pat. Nos. 5,708,247 and 5,951,836; reversing the polarity of the applied current, as disclosed in WO 01/57510; and by methods that maximize the inherent resistance of the sample, as disclosed in U.S. Pat. No. 5,628,890.
Multiple methods of applying the electric potential to the strip, commonly referred to as pulse methods, sequences, or cycles, have been used to address inaccuracies in the determined analyte concentration. For example, in U.S. Pat. No. 4,897,162 the pulse method includes a continuous application of rising and falling voltage potentials that are commingled to give a triangular-shaped wave. Furthermore, WO 2004/053476 and U.S. Publication Nos. 2003/0178322 and 2003/0113933 describe pulse methods that include the continuous application of rising and falling voltage potentials that also change polarity.
Other conventional methods combine a specific electrode configuration with a pulse sequence adapted to that configuration. For example, U.S. Pat. No. 5,942,102 combines the specific electrode configuration provided by a thin layer cell with a continuous pulse so that the reaction products from the counter electrode arrive at the working electrode. This combination is used to drive the reaction until the current change verses time becomes constant, thus reaching a true steady state condition for the mediator moving between the working and counter electrodes during the potential step. While each of these methods balances various advantages and disadvantages, none are ideal.
As may be seen from the above description, there is an ongoing need for improved electrochemical sensor systems, especially those that may provide increasingly accurate determination of the analyte concentration in less time. The systems, devices, and methods of the present invention overcome at least one of the disadvantages associated with conventional systems.